The overall goals of this research are to understand in more detail the molecular basis for the restricted specificity of human alpha-thrombin by identifying and mapping thrombin active site binding regions involved in catalytic specificity and biological recognition. In particular we wish to carry out mapping-topography studies of human thrombins in order to determine distances, overlaps and structural links between various active site loci. Of particularly critical importance will be the pursuit of structural information pertaining to the coagulant ability of thrombin, that is, the fibrinogen recognition (binding site). We have available for study the proteolytically digested four chain form, gamma-thrombin, a non-coagulant thrombin which retains complete esterase and other small substrate activities. Structural comparisons between alpha- and gamma-thrombin serve as excellent experimental-control systems for examining specifically the fibrinogen interaction. Conformational comparisons with other sequence homologous pancreatic serine proteases of known three-dimensional structures (e.g., trypsin) serve also as excellent "models" in solution for this yet crystallographically unsolved bioregulatory enzyme (alpha-thrombin). The experimental approaches will involve electron spin resonance (spin labels), nuclear magnetic resonance, fluorescence spectroscopy and general kintic and enzymological approaches.